The present invention relates to a pressing arrangement in general, and more particularly to a pressing arrangement to be used for compressing refractory or ceramic material into tile preforms.
Pressing arrangements of the type here under consideration are already known and some types thereof have already been employed in the production of ceramic or refractory tiles. One of such types of pressing arrangements includes a base from which at least two parallel uprights or beams extend upwardly. A stationary platen forms the base and a stationary support, such as a transverse beam or a similar structure, is mounted on the upper ends of the upright beams. A mold is affixed to the base intermediate the upright beams and has a set of adjacent depressions of square or rectangular cross sections. During the operation of the pressing arrangement, powdery or granulated material is first charged into the depressions by means of a charging conveyor of a conventional construction, then the material in the depressions is compressed by means of, and during the lowering of, a vertically movable horizontally extending transverse beam or a similar structure which is guided on the upright beams and has pressing projections that are directed toward and aligned with the respective depressions. Each of the pressing projections is so positioned as to enter a respective depression of the mold when the movable platen or a similar structure is pressed downwardly by a mechanically or hydraulically operated arrangement, in order for the pressing projections to compress the material contained in the depressions of the mold.
In many presses of this type which have been built recently, the pressing arrangement, which may be constituted by a hydraulic cylinder-and-piston unit or a threaded spindle rotated by a flywheel, is separate and independent from the arrangement which vertically displaces the movable platen during its approach and withdrawal from the final pressing position so that the pressing arrangement is employed only for transmitting such controlled and successive forces to the movable platen which are necessary for the actual compression of the material in the mold. Usually, each pressing operation includes a first stage of light pressing action during which the force or forces exerted on the movable platen is or are limited to the value needed for expelling air inclusions from the material to thereby reduce the thickness of the material in the respective depression of the mold to almost its final value. In other words, the pressing projections perform the greater part of their pressing stroke during such first stage, thus substantially compacting the material. Thereafter, the movable platen is subjected, during a second stage, to a stronger pressing action so that the pressing projections harden and compact the material in the respective dies into the shape of tile preforms or the like.
In such conventional presses, the pressing arrangement exerts a bias on the movable platen which is concentrated in the region of an axis of symmetry of the movable platen which extends in the direction of movement of the latter. The mold opposes such an applied bias by reaction forces which are substantially spaced from such a vertical axis so that the reaction forces cause the movable platen to elastically deform or deflect. As a consequence of this, the pressure of the respective pressing projections on the material in the respective depressions gradually decreases from the center to the periphery of the mold. These different pressures result in different degrees of compaction of the material, not only from die to die, but also from the side more distant from, to the side nearer to, the symmetry axis, within the same depression.
These differences in the degree of compaction of the material cause disadvantgeous results, especially after the tile preforms have been baked in order to obtain the tiles, inasmuch as such tiles not only have a different color from one side to the opposite one, but also have a different porosity and a different shrinkage ratio at various regions thereof so that the tiles become slightly trapezoidal in shape instead of square or rectangular, during the baking operation.
In an attempt to eliminate such negative effects, it has been heretofore proposed to make the movable platen of sizable dimensions, and particularly of a considerable cross section. When this approach is taken, the movable platen has a considerable weight. Furthermore, to limit the degree of flexion, at least that dimension of the movable platen which extends between the beams is reduced which has the advantageous effect of reducing the degree of bending of the movable platen, but also the disadvantageous effect of reducing the distance at which the beams can be located from one another. However, even this approach is not entirely successful because it only reduces but does not eliminate the flexion of the movable platen, and additionally increases the weight of the movable platen which, in turn, calls for the employment of more powerful means for displacing and biasing the movable platform. An additional drawback of this conventional type of pressing arrangement resides in the fact that the output thereof is quite small, which is attributable to the reduction of the space between the uprights available for accommodating the mold with the depression therein. To give an example, when a press of this type is used for manufacturing square tile preforms having dimensions 15 .times. 15 cm, the maximum numbers of depressions which can be provided in the mold is five. Of course, when the tiles or tile preforms to be formed in the conventional press of this type have to have larger dimensions, the number of tiles or tile preforms which can be manufactured during one complete pressing cycle is further reduced, such as to two, thus further drastically reducing the output rate of the pressing arrangement.